This invention relates to enameling compositions used as an insulating coating material for transformer wire. One of the problems involved with current methods and materials used in manufacturing and applying transformer wire coatings is the need for a suitable solvent for dissolving the coating constituents and providing a low viscosity solution. Since the solvents currently utilize expensive hydrocarbon and cresols, other materials are continuously being evaluated in an attempt to reduce the quantity of solvents employed. Over the past 15 years for example, an insulating coating composition consisting of a mixture of polyvinyl acetal and phenolic were reduced in solvent content from 85 weight percent down to 75 percent. This reduction was realized by variations in the polyvinyl acetal and phenolic materials as well as a selected combination of hydrocarbon and cresol solvents. Besides the expense involved in utilizing liquid solvents in the wire coating industry, requirements are now being made by the Environmental Protection Agency to reduce solvent usage by a substantial amount in order to reduce the overall concentration of solvents existing in the atmosphere.
U.S. Pat. application Ser. No. 595,034 filed July 11, 1975, now abandoned, discloses a three component wire insulating composition which includes an epoxy resin in combination with polyvinyl acetal and phenolic resins. The three component composition further reduced the solvent content down to 70 weight percent by taking advantage of the good filmforming properties of the epoxy resin. The use of an epoxy wire coating per se has not heretofore proven feasible due to the poor hydrolytic stability existing with known epoxy compounds. When transformer wires are coated for electrical insulating purposes, and are subjected to long exposure times in the presence of heat and moisture, it is essential that the coating remain electrically stable. Hydrolytic stability therefore is an important parameter for evaluating efficient transformer wire insulating materials. In order to determine hydrolytic stability, the transformer wire coatings are subjected to moisture and temperature for a prescribed period of time and are subsequently measured to determine whether the electrical insulating properties have deteriorated. Wires coated with epoxy compounds per se become hydrolytically unstable and are infeasible for long term transformer wire coatings.
Another requirement for transformer wire coating materials is a low dissipation factor. Since the electrical properties of the coating depend to a large extent upon the transformer operating temperature, the wire coating materials must be able to withstand the high temperatures involved, under short circuit load conditions. In order for the transformer wire coating to be electrically and thermally stable, the dissipation factor, which is a fairly good indication of the ability of the coating to dissipate heat, must be determined at various operating temperatures. If the transformer wire coating has too high a dissipation factor, thermal runaway can occur causing insulation to decrease to an inoperable value.
Formulations intended for use as insulating coatings must be carefully evaluated for temperature, moisture and overall electrical stability for long periods of time in order to ensure that short circuits do not occur due to electrical insulation failure. The polyvinyl acetal and phenolic composition disclosed within the aforementioned U.S. Patent application contains approximately phenolic resin in a ratio of one part to two parts polyvinyl acetal. Attempts to increase the polyvinyl acetal concentration resulted in wire coatings having too high a dissipation factor along with hydrolytic instability. Attempts on the other hand to increase the phenolic content seriously interfered with the flexibility of the coating. As described earlier, various epoxy resin compositions provided good flexible and pin hole free insulating coatings but were hydrolytically unstable and unsuitable per se as wire coatings. Attempts to combine epoxy resins, phenolic resins, and polyvinyl acetal such as suggested within U.S. Pat. No. Re 25,625 have not proven successful when evaluated for transformer wire coatings. Coatings prepared from the aforementioned re-issued patent disclosure were too inflexible to withstand the transformer winding operation. Wire coatings prepared from the adhesive composition disclosed within U.S. Pat. No. 3,239,598 resulted in wire coatings having an excessive dissipation factor and poor flexibility.
The three component coating composition disclosed within aforementioned U.S. Patent application Ser. No. 595,034 resulted in wire coatings having good flexibility, low dissipation factor and hydrolytic stability. The addition of epoxy resin to the polyvinyl acetal and phenolic resins substantially improved the flow properties of the coating during the coating process. With ratios of polyvinyl acetal to phenolic from about 2 to 1, additions of about 11 to 25% epoxy resin can be employed. It has since been discovered that even better coatings can be obtained with further increases in the amount of epoxy added to the coating composition. The higher epoxy compositions allow the coating to flow more evenly over the wire surface when applied by dry electrostatic techniques and, is a valuable feature in transformer wire coating operations. These higher epoxy compositions permit the formulation of higher solids enamels which greatly reduce the solvent required.
The purpose of this invention therefore is to provide three component wire coating composition having a high concentration of epoxy resin.